how to use string indexed interface of typescript?

Using the indexer limits what can be put or fetched for the object using the index syntax. E.g. foo is inferred to be of type string:

interface IDictionary {
     [index: string]: string;
}
var params = {} as IDictionary;

params['heart'] = 'HP'; // okay
var foo = params['heart']; // foo:string

The following on the other hand is an error as it will not type check:

var bar:number = params['heart']; // ERROR
params['heart'] = 234; // ERROR

Complete demo:

interface IDictionary {
     [index: string]: string;
}
var params = {} as IDictionary;

params['heart'] = 'HP'; // okay
var foo = params['heart']; // foo:string


var bar:number = params['heart']; // ERROR
params['heart'] = 234; // ERROR